1. Field of the Invention
This invention relates generally to switching power converters, and more particularly to buck-boost switching converters.
2. Description of the Related Art
A switching converter receives an input voltage VIN and produces an output voltage VOUT. One type of switching converter known as a ‘buck-boost’ converter is capable of operating in either a buck mode in which the output voltage produced is less than the input voltage (VOUT<VIN), a boost mode in which VOUT>VIN, or in a buck-boost mode in which VOUT is nearly equal to VIN.
One suitable application of a buck-boost converter is in a battery-powered system, in which input voltage VIN decreases over time; this is illustrated in FIG. 1a. Input voltage VIN might be provided by, for example, a lithium-ion battery, the voltage of which decays from 4.2 v to 2.5 v over time. If the circuitry being powered by the switching converter requires an operating voltage of, for example, 3.3 v, it is beneficial to use a buck-boost converter. When the battery voltage is above 3.3 v by a minimum amount, the converter operates in buck mode. Similarly, when the battery voltage is below 3.3 v by a minimum amount, the converter operates in boost mode. However, if the battery voltage is just slightly above 3.3 v, the converter is typically unable to continue working in buck mode, and must switch to buck-boost mode operation. Similarly, boost mode typically cannot be sustained when the battery voltage is just slightly below 3.3 v, requiring the converter to switch to buck-boost mode operation.
FIG. 1b shows an “H-bridge” circuit that is commonly used as the final stage of a buck-boost power converter. Two switching elements 10, 12 are connected together at a node 14, between VIN and a circuit common point, and two switching elements 16, 18 are connected together at a node 20, between VOUT and the circuit common point. An inductor L is connected between nodes 14 and 20, and a filter capacitor C is typically connected between VOUT and circuit common. In buck mode, switching element 16 is closed and switching elements 10 and 12 are switched on and off in complementary fashion, typically with pulse-width modulated (PWM) signals, to produce a desired VOUT. In boost mode, switching element 10 is closed and switching elements 16 and 18 are switched in complementary fashion to produce the desired VOUT. However, in buck-boost mode, all four switching elements must be switched to produce the desired VOUT.
A buck-boost mode is conventionally used because a direct transition between buck mode and boost mode can cause a discontinuity in the output voltage. There is a drawback to this method of operation, however: due to the need to be switching all four switching elements, efficiency is poor when operating in buck-boost mode.